Are the [6]-coordinated sites in tourmaline in certain cases partially vacant?

Tourmaline has two different [6]-coordinated sites, the Y site and the Z site. Vacancies were reported from both sites. Based on high-quality chemical and single-crystal structural data it usually needs increasing proportions of short-range order configurations Na(Al)Al(BO)[SiO](OH)(OH) or Na(Al)Al(BO)[SiO](OH)F in order to produce Y-site vacancies (with being the symbol for a vacant site).

Cu- and Mn-bearing tourmalines from Brazil and Mozambique: crystal structures, chemistry and correlations.

Cu- and Mn-bearing tourmalines from Brazil and Mozambique were characterised chemically (EMPA and LA-ICP-MS) and by X-ray single-crystal structure refinement. All these samples are rich in Al, Li and F (fluor-elbaite) and contain significant amounts of CuO (up to ~1.8 wt%) and MnO (up to ~3.